1. Field of the Invention
This invention relates to improved methods and compositions for the achievement of material coloration using particle scattering, as well as articles employing these material colorations.
2. Description of the Prior Art
In the prior art it is well known to color materials using dyes and pigments. Unfortunately, pigment and dye coloration agents suffer fading effects due to exposure to ultraviolet light, ozone or bleach. The usual cause of this fading is chemical changes in the colorant. These chemical changes alter the electronic transitions of the colorant, thereby causing undesired instability in color. For example, anthraquinone-based blue dyes fade upon exposure to ozone. Since most dyes contain a blue component, blue-fading causes fading in virtually every color.
It is very common in commerce to color polymers, however, colored polymer, such as dyed polymers are also difficult to recycle. Most high-value end-use applications require the separation of recycled plastics by color. However, due to the many colors available, separation by color is rarely done. Instead, most recycling companies separate plastics into a colorless lot and a mixed-color lot. Since dye removal is energy intensive, costly, and causes waste disposal problems for the spent dye, colored plastics remain in the mixed-color lot. Recycled polymer from this lot produces a marbled polymer or, as best, an off-green or brown polymer which has limited usefulness. Due to these difficulties, many recycling facilities do not even collect colored plastics. Those that do accept colored plastics often incinerate the mixed-color lot. Unfortunately, in some applications, such as carpets, almost all of the material available for recycling is colored. As a result, these polymers are rarely recycled. Consequently, billions of pounds of used carpets are discarded in landfills each year, thereby wasting valuable natural resources.
The use of chemical colorants, such as pigments or dyes, also potentially poses problems related to pigment toxicity and waste stream management. Many pigments contain toxic, heavy metals. A wet-dyeing process produces spend-dye baths. This dye-house effluent can have a negative environmental impact. The range of achievable optical effects is also restricted if the only colorants are dyes and pigments. A new technology is needed which will address fading, recyclability, dye-house effluent, and toxicity.
It would be advantageous to provide improved methods of coloration that provide switchability from one color sate to another. Such color changing compositions can be used, for example, for cosmetic purposes in polymer fibers used for textiles and carpets and for color-changing windows and displays. Additionally, this type of technology could be used in military applications for camouflage clothing, tents, and machinery. If such color change is reversibly switched as a consequence of light exposure, temperature changes, or humidity changes, then chameleon effects can be achieved for such articles. If the color switching effect is a one-time event caused by actinic radiation or high temperature exposure, the switching effect can be used to provide spatially dependent coloration.
Enhancing the value of polymer films, fibers, coatings, and other articles by achieving novel optical effects provides a major commercial goal. One advance in this area is described in U.S. Pat. No. 5,233,465 which provides a polymer film having metallic luster resulting from the multiple layering of colorless polymers having differing refractive indices. These films and derived fibers are presently used for cosmetic purposes in many applications, such as for product packaging and textile articles. Another advance is provided by the formation of a parallel-line relief pattern on the surface of a polymer film. This also results in chromatic effects without the use of dyes or pigments. A technology of this type in which the parallel-lines relief pattern consists of prisms is described in U.S. Pat. No. 4,805,984. Such polymer films are available commercially for solar window and light conduit applications.
The embossing of polymer films, especially metallized polymer films, to achieve novel optical effects is also well known. U.S. Pat. No. 4,886,687 describes non-pigmented coloration as a result of diffraction effects originating from an embossed pattern having 5,000 to 100,000 lines per inch (corresponding to a periodicity of about 0.25 to 5 microns). While such embossing provides striking visual effects for either films or film strips, such effects are difficult to perceive for polymer fibers having small diameters and conventional fiber cross-sections. Also, the embossing described in U.S. Pat. No. 4,886,687 is described to be preferably holographically generated by the interference of two coherent light beams. While such an embossing method can provide high reliability of the fidelity of the embossing process, it is also quite expensive.
Novel optical effects in silicate glasses have been achieved using colloidal particles of metals. U.S. Pat. No. 4,017,318 describes glass articles that, after exposure to actinic radiations, can be heat treated to provide coloration effects because of colloidal silver particles. U.S. Pat. Nos. 2,515,936; 2,515,943 and 2,651,145 also describe methods of generating colored silicate glasses using combinations of various colloidal metals, including colloidal gold and silver. Pearlescent compositions are also widely used to provide novel optical effects, including color, to polymer articles. These compositions, such as described in U.S. Pat. Nos. 3,087,829 and 4,146,403, provide coloration due to the interference of light reflected from parallel opposite sides of platelets deposited on the plate sides of micasubstrate particles. This interference-derived coloration process critically depends upon the nearly perfect parallel arrangement of the reflecting surfaces of plates. Hence, such colorants are sometimes referred to as plate interference colorants. Due to the many micron diameter of the plates, such particles are unsuitable for the spinning of fibers of the types conventionally used for textiles and carpets, since the available pearlescent platelets have lateral dimensions that are comparable to the diameter of the such fibers. As a result, these platelets are either filtered out during the fiber spinning process or they clog spinneret holes.
The pearlescent platelets are preferably aligned parallel to the polymer surface. Without such parallel alignment, the color effect is not as dramatic. Additionally, thick polymer articles are required in order for the iridescence to be pronounced at the loading levels that can be used without severely degrading polymer mechanical properties.
Christiansen filters have been known for over a century. Such filters usually consist of particles of a solid in a liquid matrix. The particles and the host matrix are chosen so that the wavelength dependence of the refractive index of the host matrix and particles are substantially different and there exist a wavelength at which the refractive index of the host matrix and the particles are equal. At that wavelength the filter is transmissive and at wavelengths remote from that wavelength the light is largely scattered and not transmitted. For effective operation at visible wavelengths, such filters should not contain components that significantly absorb light at these wavelengths. U.S. Pat. No. 3,586,417 shows that the wavelength at which a Christiansen filter transmits can be varied for an optical device by varying the temperature of the filter. Such variation results from the different temperature coefficients for the refractive indices of the scattering particles and the liquid matrix. Various new methods for producing Christiansen filters, including some efforts to make solid-matrix optical devices, are described by Balasubramanian, Applied Opties 31, pp. 1574-1587 (1992). While Christiansen filters are very useful for providing wavelength-selective light transmission for optical applications, means for obtaining specially enhanced coloration effects for scattered light using the Christiansen effect have not been previously demonstrated. Such enhanced effects for scattered light have critical importance for the development of new technologies for achieving material coloration.
The present invention eliminates the above described problems of prior art technologies by the use of coloration associated with particle scattering. Materials and methods for modifying and enhancing the coloration effects of particle scattering are provided by this invention.